The invention relates in general to polarization properties of antennas. In particular the invention relates to adjusting the polarization properties of an antenna based on the polarization properties of a received signal.
Electro-magnetic radiation has certain polarization properties. When receiving signals carried by radio frequency (RF) electro-magnetic radiation, generally the polarization properties of the antenna are matched to those of the incoming signal. If electro-magnetic radiation experiences a reflection, its polarization properties typically change. Therefore there may be need to adjust dynamically the polarization properties of an antenna depending on the path of the radiation from the transmitting antenna to the receiving antenna. Otherwise it is, in the worst case, possible that the polarization properties of an antenna are orthogonal to those of the electro-magnetic radiation that is received using the antenna. In this case the antenna cannot detect the electro-magnetic radiation.
The Global Positioning System (GPS) is used here as an example of a system, where there is need for adjusting the polarization properties of the receiving antenna. GPS is a positioning system, where a receiver device can compute its position using signals it receives from GPS satellites. The GPS system has two services: Standard Positioning Service (SPS) is available for all users and Precise Positioning Service (PPS) is available, for example, for certain military users. Each GPS satellite sends the two positioning signals which are spread spectrum signals. A Coarse Acquisition (C/A) code and a Precise (P) code are modulated to carrier frequencies of 1575.42 MHz and 1227.6 MHz using binary phase shift keying. The C/A code, for example, is a pseudorandom binary code consisting of 1023 chips and it is repeating itself every millisecond. The chip rate of the C/A code is thus 1.023 MHz. The C/A and P codes are GPS satellite specific. The satellites send also navigation information at the data rate of 50 bps.
The GPS transmitters in satellites transmit the digital positioning information using right-handed circularly polarized (RHCP) radiation. When receiving GPS signals, the GPS receiver is usually outdoors and has a direct line of sight (LOS) connection with the GPS satellite. In a LOS connection the received signal (or at least a certain part of the received signal) is not reflected, and the receiver antenna may have the same polarization characteristics as the transmitted radiation.
FIG. 1 presents by the way of example a schematic drawing of the antenna 101 and receiver 110 of a GPS receiver device 100. The antenna 101 is typically a RHCP antenna. An output from the antenna 101 is connected via a preamplifier to the RF part 111 of the receiver 110. In the RF part 111 the received broadband signal is usually first filtered using a band-pass filter, and the result is a RF signal. The RF part further has a local oscillator (LO) and the RF signal is typically mixed with the sinusoid produced by the LO to produce an intermediate frequency (IF) signal. This IF signal, which typically comprises an I (In phase) component and a Q (Quadrature phase) component, is delivered an analog-to-digital (A/D) converter 112, where the signal is sampled. The sampled signal is processed in a Correlator 113, where a local copy of a C/A code is correlated with the C/A code using which the navigation data has been spread. In this correlation process the correct C/A code and its phase is found. This information about the C/A code presented in FIG. 1 with arrow 121. It is also possible that a correlator can be placed before the A/D converter 112. The correlated digital signal is output to a Digital Signal Processing (DSP) unit 114. This DSP 114 is responsible, for example, for determining which radio symbols are sent. There is a mapping between the symbols and the data bits, so the output from the DSP is the navigation bit stream (arrow 122). The GPS position block 115 of the GPS receiver 100 takes at least the C/A code information (arrow 121) and the navigation bit stream (arrow 122) as input. Typically position determining needs information about the phase of at least three C/A. There can be, for example, three correlators 113, and each is correlating one C/A code with the IF signal, or one correlator 113, which correlates various C/A codes to the signal alternately.
If a GPS receiver is used indoors, typically for example near a door or a window, the received radiation carrying positioning information may be reflected. Reflected right-handed circularly polarized radiation is left-handed circularly polarized radiation. An antenna in a GPS receiver, which is optimal for receiving right-handed circularly polarized radiation, may not detect a left-handed circularly polarized radiation at all, or at least the intensity of the detected signal may be orders of magnitude less than that of the actual signal. Therefore a GPS receiver that is to be used indoors is preferably able to receive efficiently also LHCP radiation. In certain cases, where some of the received GPS signals are reflected and others are not, a GPS receiver should be able to receive at a time LHCP and RHCP signals.
It is generally known that electro-magnetic radiation having any polarization properties can be decomposed into two linear components with certain relative phase and having certain relative amplitudes. Using, for example, two linearly polarized and orthogonal antennas it is possible to receive electro-magnetic radiation having any polarization properties. The problem may, however, be the dynamical changes in the received radiation. When a user carrying a GPS receiver moves, the path of the GPS signal from the satellite transmitter to the GPS receiver may change and, consequently, the polarization properties of the received GPS signal may change.
Schemes for adjusting the properties of an antenna according to the properties of the received signal have been proposed. One alternative is to modify the physical properties of the antenna, but this may require quite complex mechanical arrangements and most probably the modifications cannot be made at a high rate. A second alternative is to use two antennas having different polarization properties and to combine the signals received using these two antennas. Patent application EP 416 264, for example, discusses a system which uses two orthogonal linearly polarized antennas and which is implemented using analog components. The two received signal components have different phases and amplitudes, and a calibration circuit detects the phase and amplitude differences of the received signal components. The phase and amplitude differences are used to adjust variable phase shifters in a combining circuit where 90° hybrid couplers are used to combine the signal components. The calibration circuit continuously compares to signal components, and the signal components are passed through delay lines to the combining circuit. The delay allows the variable phase shifters to be adjusted before the signal components enter the combining circuit.
A similar system could be used in a GPS receiver, but this would require two linearly polarized antennas in the GPS receiver and a calibration circuit and a combining circuit should be built in the RF part (corresponding to the RF part 111 in FIG. 1) related to each antenna. Additional components increase the cost and manufacturing complexity of a device. Furthermore, if the GPS receiver is intended to be integrated within a cellular phone, the number of components should be kept as low as possible because of power consumption and space requirements. A further problem is that with the system presented in EP 416 264, a GPS receiver, which is designed to work indoors, should have more than two antennas: a pair of orthogonal antennas is needed for each GPS signal that is to be received at a time.
Polarization diversity is a known method to increase the quality of a received signal. FIG. 2 presents an example of a polarization diversity receiver 200, which has two antennas 101a and 101b having different polarization properties are used to receive a signal. Each antenna is connected to its own RF block, and the RF blocks 111a, 111b are, in turn, connected to A/D converters 112a, 112b. The digitized signals {tilde over (S)}1 corresponding to the signal component received with the first antenna 101a and {tilde over (S)}2 corresponding to the signal component received with the second antenna 101b are complex-valued signals, and they are processed further in the DSP block 201. When polarization diversity is employed, either one of the received signals is used in the reception or both received signals are properly combined and used in the reception. A GPS device employing polarization diversity would typically have two antennas having orthogonal circular polarization properties, so it would have two antennas 101 and two receivers 110. In outdoor environment it would typically employ only the RHCP antenna and the receiver corresponding to the RHCP antenna.